Our 2013 GMC is a 2500HD. Before we left the United States we completed some modifications to the truck to beef up the suspension. Airbags were added to help keep the load level. All four shocks were replaced, which helped with the porpoising or bouncing of the rig. A sway bar was installed on the rear axle to reduce the side-to-side movement of the truck. We also bought E-rated tires. These items really helped with the ride of the load. 
The one difference between our 2500HD and a 3500 is one additional leaf spring. This is something that Mike has wanted to add for a while, but time and money made us hesitate. While we were in Oaxaca City, we found both and got the work done.
Near the edge of the city is a shop named REMSAC. They were listed on iOverlander with good reviews from other travelers. We stopped by and talked to them about adding a new leaf spring. It was clear that many large trucks count on this shop to handle their suspension needs, as the lot was full. The appointment was set and a price of $200usd was set. The next day we arrived early and pulled the truck over the pit and slid it in with the big boys for the day. Then we took the dogs and headed out for a day in the city. 
When we returned around 5:00 the leaf spring was done and the truck was ready to go. In one workday they had fabricated and installed a custom leaf spring on the suspension. The spring is in, and the ride feels a little stiffer. But it also feels more stable. It is the one with the shiny edge in these photos. The pair of leaf springs cost us just under $200usd.
The next day we had an appointment for brakes, fluids and a general check up at a different mechanic. This shop was very neatly organized 
